The present disclosure relates to a gas turbine engine and, more particularly, to a combustor therefor.
Gas turbine engines, such as those which power commercial and military aircraft, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. The combustor generally includes radially spaced apart inner and outer liners that define an annular combustion chamber therebetween. Arrays of circumferentially distributed combustion air holes penetrate multiple axial locations along each liner to radially admit the pressurized air into the combustion chamber. A plurality of circumferentially distributed fuel injectors axially project into a forward section of the combustion chamber to supply the fuel for mixing with the pressurized air.
Combustion of the hydrocarbon fuel in the presence of pressurized air may produce nitrogen oxide (NOX) emissions that are subjected to excessively stringent controls by regulatory authorities, and thus may be sought to be minimized.
At least one known strategy for minimizing NOX emissions is referred to as rich burn, quick quench, lean burn (RQL) combustion. The RQL strategy recognizes that the conditions for NOX formation are most favorable at elevated combustion flame temperatures, such as when a fuel-air ratio is at or near stoichiometric. A combustor configured for RQL combustion includes three serially arranged combustion zones: a rich burn zone at the forward end of the combustor, a quench or dilution zone axially aft of the rich burn zone, and a lean burn zone axially aft of the quench zone.
During engine operation, a portion of the pressurized air discharged from the compressor enters the rich burn zone of the combustion chamber. Concurrently, the fuel injectors introduce a stoichiometrically excessive quantity of fuel into the rich burn zone. Although the resulting stoichiometrically fuel rich fuel-air mixture is ignited and burned to release the energy content of the fuel, NOX formation may still occur.
The fuel rich combustion products then enter the quench zone where jets of pressurized air radially enter through combustion air holes into the quench zone of the combustion chamber. The pressurized air mixes with the combustion products to support further combustion of the fuel with air by progressively deriching the fuel rich combustion products as they flow axially through the quench zone. The fuel-air ratio of the combustion products changes from fuel rich to stoichiometric, causing an attendant rise in the combustion flame temperature. Since the quantity of NOX produced in a given time interval increases exponentially with flame temperature, quantities of NOX may be produced during this initial quench process. As the quenching continues, the fuel-air ratio of the combustion products changes from stoichiometric to fuel lean, causing an attendant reduction in the flame temperature. However, until the mixture is diluted to a fuel-air ratio substantially lower than stoichiometric, the flame temperature remains high enough to generate quantities of NOX.
Low NOx combustor designs stabilize the primary combustion zone with a swirling flow and cooling jets through combustion holes close to this zone. To assist in primary zone stabilization, the combustor configuration also may have a bulged contour. Subsequent to these designs, improved air-blast injectors with one or two rows of unopposed dilution jets were provided for rapid mixing. Trends to decrease residence time with further NOx reduction continued with increasingly strong dilution jets. From the data acquired to-date through engine testing, demonstration and certification requirements, the stability for primary zone combustion followed by (close to) stoichiometric combustion are directly related to (1) the mixing characteristics of fuel-air injectors, (2) aerodynamic contouring of the combustion chamber, and (3) the dilution jets.
Combustion processes with several stages of combustion are desirable; however, a minimum length for the combustor is required, which, in turn, may result in a relatively significant weight requirement. Dilution cooling/mixing jet configurations that radially penetrate into the mixing zone with sufficient strength may also result in a quasi-one-dimensional momentum for each dilution jet prior to onset of the desired counter-swirl effect of the two jets combined. This may result in an exit temperature profile with circumferentially peaks which may expose the turbine section to excessive temperatures.